The importance of achieving highly accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such accurate alignment difficult both to achieve and to maintain. For example, one issue of concern in the construction of laser transmitters is that of efficiently coupling the optical output from an optical device such as a laser diode into an optical fiber. To obtain efficient coupling, the fiber end is desirably precisely aligned with the emitting area of the laser. When such alignment is achieved, the fiber is then fixed in place, desirably by a method that enables the precise alignment to be sustained throughout the device lifetime.
Typically, fiber-coupled diode lasers are packaged in metal butterfly packages, which may be gold plated, and the fiber is held in alignment with the laser using one of the epoxy, laser weld, or solder attachment techniques with or without a ferrule. Epoxy attachment is low cost but may have too much thermal expansion for high precision attachments. Furthermore, it may not be reliable over a long period of time due to outgassing and alignment shifts arising from aging and temperature cycling. Laser weld techniques are reliable but use costly ferrulization of the fiber and specially designed mounts or clips to allow weld attachment of the ferrulized fiber. The mounts/clips are expensive, large, and may creep over time. Solder attachment techniques, on the other hand, are reliable and low cost, and have become prevalent in the art. Existing solder attachment techniques however, tend to use an integrated heating mechanism and/or a specially configured platform to isolate the heat used for solder reflow. These thermal management means may be expensive and/or undesirably large.
Typically, precise alignment of the fiber involves aligning the end of the fiber in at least one direction relative to the optical device to provide a maximum energy transfer from the optical device to the fiber. A further optical device such as a photodiode or any light emitting diode may be used to measure an optical power coupled into the optical fiber. The fiber may be precisely aligned in at least one of a vertical and a lateral direction. The fiber may also be adjusted horizontally to minimize a gap distance between the fiber and the optical device. The fiber may be adjusted in vertical and lateral alignment until a maximum power is determined. A predetermined gap distance may be used for horizontal alignment or the gap distance may be adjusted while visually monitoring the distance to avoid direct contact between the fiber and the optical device.
It is difficult, however, to maintain alignment between the optical component and the fiber when the fiber is soldered due to turbulent flows and capillary forces exhibited by the molten solder. It is further difficult to determine the precise direction of misalignment after the fiber has been soldered.